Historical Geology/Steno's principles

Steno's principles of stratigraphy are among the oldest of geological principles; they explain how we can look at sedimentary rocks today and figure out facts about the sediment at the time of deposition.

They are named for Nicholas Steno (1638 - 1686), a Catholic bishop and polymath scientist who set forth his principles in his Dissertationis prodromus of 1669. He was beatified in 1988, making him officially just one step away from being a saint; the scientific world has shown its admiration for him by naming craters on Mars and the Moon in his honor.

In this article we shall explain, discuss, and illustrate his principles.

Contents

Beds of sandstone and siltstone: the lower strata were deposited first.

The principle of superposition simply says that when sediments are deposited, those which are deposited first will be at the bottom, and so the lower sediments will be the older. This is because sediment is deposited from above, because gravity operates in a downward direction, and because sediment does not readily pass through other sediment.

Note that this only applies to sediment; it does not necessarily apply to igneous rock, because whereas sediment is deposited from above, magma oozes up from below. So, for example, an igneous sill intruding into sedimentary rock will be younger than the rocks immediately above it.

One snag may occur to you. It is all very well to say that the sediment, when originally deposited, was laid down from the bottom up. But does it necessarily follow from this that when we look at sedimentary rocks the lowest is the oldest? For tectonic processes such as folding can completely overturn a section of rock, or, just as bad, turn it on its side so that we can't tell which was originally up and which was down.

Fortunately, there are many clues within the rocks which allow us to discover which way up they were originally; these are known as way-up structures and will be the subject of the next article. Once we have used these indications to discover which way up the rocks were when the sediment was deposited, we can then apply Steno's principle to sort out their relative ages.

The principle of original horizontality states that sediment is originally laid down flat.

This needs some qualification. After all, some sediment is not originally deposited in flat beds, as you will know from the articles on sedimentology: for example aeolian sand is laid down in cross-beds, and the angle of repose of a sand dune can be as much as 34º from the horizontal.

Cross-bedding, Dry Fork Dome, Utah

Yet you will also recall that the cross-beds are laid down in sets formed as the dune rolls across the desert landscape, and these are much more nearly horizontal. Consider the photograph to the right of aeolian sandstone in Utah.

The cross-beds themselves reflect the angle of repose of the original sand-dunes; but the sets of cross-beds are horizontal.

To take another example, you may remember that the progradation of a delta forms foreset beds, which show the slope of the delta into the lake or sea into which it builds. And yet these foreset beds considered as a whole will form a roughly horizontal layer sandwiched between the topset and bottomset beds.

So even apparent exceptions can, looked at the right way, be seen as instances of Steno's rule; and having taken them into account, we find that we can look at rock formations and identify the plane of horizontality at the time that the sediment was originally deposited.

The principle of original continuity states that sediment is originally deposited in a continuous horizontal sheet until it meets some obstacle, such as the base of a cliff, or it tapers off because of its distance from its source, or grades laterally into another sort of sediment.

The qualifications in that sentence may seem at first to make the principle futile. What is it saying but "sediment will go on until it stops"?

Hoodoos at Drumheller, Canada.

An example will make the concept clearer. Consider the photograph to the right of hoodoos at Drumheller, Canada.

You should easily be able to make out three kinds of sediment: there is hard dark mudstone at the bottom, followed by lighter-colored softer sandstone, and then the "caps" visible on the taller hoodoos are formed from sandstone which is harder and darker than that which underlies it.

Now, it is inconceivable that geological processes originally deposited these sediments in the forms of hoodoos: for one thing, there are no processes that would do that; and for another thing, the sediment, when unlithified, wouldn't have stayed standing if it had been deposited in such a way.

What must have happened is that each of the three layers was originally deposited as a continuous sheet, which subsequently underwent lithification and then erosion, producing the forms visible today.

As we have just explained the principle of actualism in the previous article, it is worth taking a while to examine how Steno's principles are just special cases of this more general principle.

For example, when discussing the processes that formed the hoodoos above, we appealed twice to actualism; we wrote: "It is inconceivable that geological processes originally deposited these sediments in the forms of hoodoos: for one thing, there are no processes that would do that, and for another thing, the sediment, when unlithified, wouldn't have stayed standing". We appealed to what we know of geological processes to say that the hoodoos couldn't have been deposited as hoodoos; we appealed to known laws of physics to point out that even if there was such a process, the unlithified sediment would have slumped and collapsed.

Again, consider the principle of superposition as it applies to the hoodoos. It seems plain that mud must have been deposited first, then the paler sand, and then the darker sand. In saying so, we are tacitly taking it as axiomatic that science-fictional processes such as levitation or teleportation did not come into play. If they did, then we have no way of knowing which rock layer came first: maybe the dark sand was deposited first but hung unsupported meters above the sea floor until the gap was filled in by the lower sediments which somehow managed to pass through the existing layer.

We can imagine such a thing happening, just as we can imagine a dragon or a unicorn, but the only basis for practicing geology is to ignore this hypothetical magical event and to work on the basis that this did not in fact happen.